Introduction

The charged current event sample is contaminated with overlapping events. These can either be due to cosmic ray or "halo" muons together with a normal e-p-interaction or beam-gas event.

Halo muons are muons created by interactions upstream of the detector, which traverse the detector from RCAL to FCAL at a large radius. These halo muons can produce large because they traverse the BCAL parallel to the scintillator plates. The pulse height registered can then be many factors larger than one would expect from a muon, due to the significantly increased sampling along the muon track.

Cosmic muons arrive predominantly from above and traverse the detector from top to bottom. These are in general no problem as the minimum ionizing energy deposit will not generate a large transverse momentum. There are exceptions to this though. A cosmic muon can enter FCAL or RCAL or to a lesser extent BCAL from above and again travel parallel to the scintillator plates, giving large apparent energy, and then in general as they are "off center" a large . Another possibility for a cosmic muon to produce large is that the high energy muon produces a bremsstrahlung photon in the UCAL, before exiting the detector. Also events with multiple muon tracks form a significant background.

Figures 6.1 through 6.3 and figure 6.8 show examples of the different background categories mentioned above.

These events can not be detected by the algorithms employed so far, as these algorithms assume that the halo muon or cosmic is alone in the detector. When the halo or cosmic muons overlay a genuine e-p event the information from the underlying e-p-event masks the information of the muon and thus render the algorithms useless.

The muon finder ( MUFFIN) described in this chapter attempts (and succeeds) to find these muons. The principle of the program is to search for a topology of calorimeter cells within normal events, which is consistent with a muon traversing the detector. The characteristic topology is a series of aligned cells (i.e. high energy muons traverse the detector in straight or almost straight lines). When a muon candidate is found the event is removed from the CC sample if by removing the muon candidate energy deposits the event no longer passes the CC selection cuts. The detector elements that are important in the muon finder are the muon chamber system ( FMUON, BMUON and RMUON), the calorimeter ( FCAL, BCAL and RCAL) and the central tracking system ( CTD).

In current physics analyses visual scanning is an accepted method to remove this source of background. This method has several disadvantages though:

• The number of events a physicist can scan for an analysis is limited.
• Visual scanning has a high efficiency but the efficiency is not constant.
• It is difficult to estimate the efficiency and purity of the visual scan method.

The following will be a detailed description of the muon finder program.

Figure 6.1: Example of a cosmic muon traversing the ZEUS detector together with what appears to be a beam-gas event. On the left side the cells surrounding the FCAL beam pipe region are shown. On the right side the cosmic muon which traverses the BCAL can be seen. In the middle of the picture tracks stemming from the beam-gas event are drawn.

Figure 6.2: Example of a halo muon traversing the ZEUS detector all the way from RCAL through FCAL together with what appears to be a beam-gas event, originating inside the detector volume.

Figure 6.3: Example of a cosmic muon traversing the ZEUS detector. The muon hit cells in RCAL, BCAL and FCAL. Its trajectory passes through the beam-pipe. The reason that this event was identified and rejected by MUFFIN is that the timing of the two condensates was compatible with a particle traveling between them.